The present invention relates to a display means making it possible to observe a synthetic image which is collimated and superimposed on the natural view of the landscape. These means, which are generally used on board aircraft, are often called head-up displays or HUD to distinguish them from the head-down display of means provided on the instrument panel.
The invention is more particularly intended for airborne use, particularly for use in a helicopter, in order to provide both a firing sight unit and a flying instrument, i.e. the flight collimator.
These means generally comprise an image combining optical system for transmitting to the observer the radiation coming from the landscape and for reflecting to the observer the radiation corresponding to a synthetic image to be displayed. The latter is produced by an optical collimator incorporating an optical lens and bright objects representing the symbols to be displayed and which are positioned in the focal plane of the lens. For this purpose, the bright object generating means generally use a cathode ray tube, the figuring of the symbols represented on the screen being reflected to infinity by the lens.
According to conventional solutions, the combining optics comprises a partly transparent glass plate, or several parallel plates for increasing the optical pupil and consequently the instantaneous field without changing the diameter of the collimating optics.
In such equipment, the cathode ray tube with the associated circuits (supply, scanning, etc) and the collimating optics are placed in a box or case, which is also mechanically connected to the image combining optics. This case as well as the mechanical mounts or pillars supporting the combining optics provide a mask, which can be of considerable size, due to the overall dimensions of the case. The effect of large mask (hereinafter, mask effect) can be very operationally prejudicial and dangerous if it is located in the field of vision of the observer.
Moreover, from the aircraft side, helicopters are essentially characterized by a large canopy, and flying conditions (particularly tactical flying conditions) in which the mask introduced by a head-up display means must be minimized in size, even when installed in the upper part of the cabin.
The object of the invention is to provide a head-up display meeting these requirements, whilst being adapted to the configuration of the aircraft where it is to be installed, in such a way that the mask produced by it almost completely coincides from the observer's standpoint with a natural mask existing in the cabin, which means that there is a virtually maskless display, which does not limit the field of observation. The display is more particularly intended for installation in the upper part of the cabin.
According to the known display techniques, a spherical reflector forms the collimating optics. In accordance with the so-called "on the axis" use, adjacent to the reflecting spherical optics is located a partly reflecting glass plate, which combines the images. According together quasi-axial solutions corresponding holographic optics are used having a first diffracting element in the form of a spherical glass and at least one second diffracting element formed by a flat glass, the assembly ensuring the collimation and image combination function.
A system of this type is described in European Patent Application No. 0,009,332. The head-up display is intended for operation in a low position, the cathode ray tube being installed on the instrument panel. The optical unit constituted by the holographic optical elements is essentially placed in the visual field of observation and is rigidly fixed by the bottom and laterally to the case and to the instrument panel.
It is not really possible to transpose this solution as such for the requirements of head-up operation, particularly on board a helicopter, in view of the considerable mask effect. The optical module comprises the spherical diffracting element and the planar diffracting element assembled so as to have in cross-section a substantially triangular structure, each forming a side of a triangle, whose third side is constituted by a mechanical supporting element or part of the case. This solution does not permit an on the axis use, the radiation from the cathode ray tube having to pass through the spherical mirror and then has to be reflected twice, by the flat mirror and the spherical mirror, before finally passing through the flat mirror.